Lyme disease, the most prevalent vector-borne disease in the United States, is caused by infection with the spirochete, Borrelia burgdorferi. Antibiotic treatment during early infection is generally curative. Untreated, the disease can progress to more chronic neurologic and rheumatologic manifestations and therefore, effective management of Lyme disease is dependent on early diagnosis. Currently, diagnosis is based on both clinical findings and serologic testing. The most commonly employed serologic assays suffer from lack of sensitivity during early disease, lack of standardization, intra- and inter-laboratory variation and a high false-positivity rate. As a result, patients are often tested multiple times and the tendency for overtesting and overdiagnosis (or misdiagnosis) is great. A long-term goal of this project is development of a novel diagnostic approach based on alterations in host gene expression induced by exposure to B. burgdorferi. Host global gene expression, reflecting responses of the host to a specific pathogen, may have the potential to provide evidence of such an infection. We hypothesize that B. burgdorferi infection induces modifications in host gene expression. Using differential gene expression analysis, we propose to identify host genes whose expression is altered by B. burgdorferi infection. This gene set can then be employed as an indicator for B. burgdorferi infection in clinical specimens. The following specific aims are proposed: 1) Variation in host gene expression on infection with B. burgdorferi will be assessed in a murine model. Mice will be infected B. burgdorferi and differences in mRNA expression between control and infected animals will be measured by microarray analysis; 2) Peripheral blood mononuclear cells (PBMCs) isolated from healthy donors will be exposed to B. burgdorferi or other potential blood-borne pathogens and differences in global gene expression will be monitored by microarray analysis; 3) Genes differentially expressed in PBMCs isolated from Lyme disease patients will be identified by microarray analysis. These studies will include patients with acute, non-disseminated early disease, early disseminated infection and Lyme arthritis. The studies proposed here should result in identification of those genes in the host whose expression is modified during B. burgdorferi infection. This information, in turn, can be used to design host gene-based Lyme disease diagnostics for the detection of active B. burgdorferi infection.